Simple FM Transmitter Working

[Aswinth Raj]

Hi,

I have been trying to build a simple FM transmitter as a hobby project and used a circuit that I found online. The circuit is given below



I know the circuit is working since someone has made it and documented it with a POC video here

https://circuitdigest.com/electronic-circuits/how-to-build-fm-transmitter-circuit

My question is how is such a simple circuit built on a breadboard works? I tried the same circuit and it is not working:bang:

I also tried everything suggested in this thread
https://www.edaboard.com/showthread.php?320858-fm-transmitter-circuit-not-working

Initially I started building this circuit on breadboard but many people say that Oscillatory circuits should not be built on breadboard so should I move to a pref board?
Also is it not okay to use a simple wire as antenna? Some people suggest for a yogi antenna? I am totally confused now:!: please guide me

Thanks

 

[betwixt]

Simple answer - yes it should work.

Detailed answer - the length of component wires and interconnections is critical. Consider the conductors on breadboard /perf board will have considerable effect on the operation of the oscillator particularly. Even a few mm of extra copper on one of the connections may alter the tuning by several MHz so finding the correct frequency is very dependent of construction. In such a simple design, which has no output buffering, the length of the antenna and it's proximity to other objects will also change the tuning. For example, you would find that moving close to the antenna shifts the frequency.

A Yagi antenna (like a TV antenna) is overkill for this design and would be difficult to connect to it.

Brian.

 

[Aswinth Raj]

Thanks Brain,

Looks like I should spend more time trying to tune in and find the frequency at which my circuit is working.

So let me go though the connections one more time and try tuning my radio again.

Will update you guys If I had any luck.

 

[Audioguru]

The capacitance between the rows of contacts and wires on a solderless breadboard are too much for the high frequency. Get rid of the breadboard and use very short wires on a perf board or stripboard instead.
I fixed that simple defective circuit years ago on another website in 4 stages:
1) The mic preamp transistor is poorly biased so the transistor is saturated when the battery is new and the transistor has high Hfe, and the transistor is cutoff when the battery is old and the transistor has low hFE. So I biased the transistor correctly and added a 5V low dropout voltage regulator.
2) The radio frequency changed when the battery voltage runs down. I powered the RF oscillator transistor from the added 5V voltage regulator.
3) The radio frequency changed when something moved toward or away from the antenna. So I added a RF amplifier transistor as a buffer between the oscillator and the antenna.
4) The sounds heard on a normal FM radio sounded muffed like the treble tone control is turned all the way down. So I added pre-emphasis (treble boost) like all FM radio stations have which is cut down to normal by the de-emphasis in all FM radios.

Here is my circuit built compactly on stripboard: